The reduction in the lead content of gasolines and the use of lead anti-knock compounds has lead to a search for other ways to improve the octane number of blending components for gasoline. The alternatives to uses of lead anti-knock compounds are chemical processing and the use of other additives. Simultaneously with the reduction in lead has come the requirement that the benzene content of gasoline must be reduced.
One common process long used by the refinery industry to upgrade raw naphtha to high octane gasoline is catalytic reforming. In catalytic reforming the raw naphtha having a boiling range of circa 115.degree.-350.degree. F. is passed over an alumina supported noble metal catalyst at elevated temperatures (circa 920.degree.-950.degree. F.) and moderate pressure (circa 200-550 psig). The catalyst "reforms" the molecular structures of the hydrocarbons contained in the raw naphtha by removing hydrogen and rearranging the structure of the molecules so as to improve the octane number of the naphtha. However, the increase in octane number also reduces the liquid volume of the naphtha as the specific gravity is increased.
Because of the multiplicity of the compounds in the raw naphtha, the actual reactions which occur in catalytic reforming are numerous. However, some of the many resulting products are aryl or aromatic compounds, all of which exhibit high octane numbers. The aryl compounds produced depend upon the starting materials which in a refinery are controlled by the boiling range of the naphtha used and the crude oil source. One of the aryl compounds formed is benzene which as noted above is not desirable. One method suggested for reducing the benzene content of the reformate is to separate the benzene precursor (isohexane) from the feed to the catalytic reformer. See for example U.S. Pat. No. 4,975,179. This expedient, however, reduces the overall octane of the gasoline mixture.
The "reformed" product from a catalytic reforming process is commonly called reformate and is often separated into two fractions by conventional distillations--a light reformate having a boiling range of circa 115.degree.-250.degree. F. and a heavy reformate having a boiling range of circa 250.degree.-350.degree. F. The aryl compounds in each fraction are thus dependent upon their boiling points. The lower boiling or lighter aryl compounds, e.g., benzene, toluene and xylenes, are contained in the light reformate and higher boiling aryl compounds are contained in the heavy reformate.
A method of carrying out catalytic reactions has been developed wherein the components of the reaction system are concurrently separable by distillation using the catalyst structures as the distillation structures. Such systems are described variously in U.S. Pat. Nos. 4,215,011; 4,232,177; 4,242,530; 4,250,052; 4,302,356; and 4,307,254 commonly assigned herewith. Briefly, a structure described there is a cloth belt with a plurality of pockets spaced along the belt containing a particulate catalyst, which is then wound in a helix about a spacing material such as stainless steel knitted mesh. These units are then disposed in a distillation column reactor. In addition, commonly assigned U.S. Pat. No. 4,443,559 discloses a variety of catalyst structures for this use and is incorporated by reference herein.
It has been proposed to alkylate the aromatics, especially benzene, contained in a reformate stream utilizing the concurrent distillation/reaction method. See U.S. Pat. No. 5,082,990. The direct alkylation of benzene with ethylene or propylene has been quite difficult due to fast catalyst deactivation. One reason for the fast deactivation is the requirement for high conversion of benzene. In the concurrent reaction/distillation the depletion of the benzene in the reaction mass increases the concentration of olefin around the catalyst leading to fast catalyst deactivation. In other types of reaction systems the coexistence of toluene and other aromatics leads to the production of a large amount of higher alkylated aromatics, depressing the vapor pressure of the reaction mixture. In addition the high non-aromatic C.sub.6 's in the reformate make the alkylation of benzene more difficult. These hydrocarbons boil in about the same boiling range as benzene and are therefore difficult to separate from benzene without affecting the conversion of benzene.
However, there is almost three times as much toluene as benzene in the reformate and the alkylation of toluene with an olefin is relatively easy.